A conventional hydraulic circuit for controlling a boom of a construction machine in accordance with the prior art as shown in FIG. 1 includes:                a hydraulic pump 1 connected to an engine (not shown);        a boom cylinder 2 connected to the hydraulic pump 1 through a first path (or a large chamber-side flow path of the boom cylinder) 2a and a second path (or small chamber-side flow path of the boom cylinder) 2b;         a boom control valve 3 shiftably installed in a flow path provided between the hydraulic pump 1 and the boom cylinder 2 and configured to be shifted to control a start, a stop, and a direction change of the boom cylinder 2;        a holding valve 6 including a holding poppet 4 installed between the boom control valve 3 and the first flow path 2a of the boom cylinder 2 and a drain valve 5 configured to supply or discharge a hydraulic fluid to or from a back pressure chamber 4a of the holding poppet 4 so that the natural descending movement of the boom due to fluid leakage, empty weight, and the like is prevented when the boom control valve 3 is in a neutral state; and        a port relief valve 7 installed in the first path 2a at the downstream side of a holding poppet 4 and configured to drain the hydraulic fluid to a hydraulic tank (not shown) when an overload occurs in the first path 2a.         
In case of such a conventional hydraulic circuit, when an overload is applied to the boom cylinder 2 in a direction in which the boom descends or is lowered in a state in which the boom is not manipulated, an overload exceeding a predetermined pressure of the port relief valve 7 is applied to the port relief valve 7 installed in the first path 2a, so that a large chamber-side hydraulic fluid of the boom cylinder 2 is drained to a hydraulic tank (not shown) through the port relief valve 7. Therefore, the conventional hydraulic circuit entails a problem in that there may occur safety accidents due to the abrupt descending movement of the boom.